[Asterisk-Users] 503 Error

[doug at simflex.com]

When trying to send calls from our Asterisk PBX to our upstream
termination provider, I am getting

Got SIP response 503 "Service Unavailable" back from PROVIDER

We are sending the calls without registration, there is no username and
password.  When we were using SER it would send them without a problem.

Can someone tell me if the problem is on my side or the provider's?

What would cause this problem?

Thanks
Doug